1. Field of the Invention
The invention pertains to underground septic tanks, more particularly to a septic tank such as a distribution tank, fluid separator tank, and settling tank, that is adapted for burial directly in the soil, and which is light weight in construction such as one molded from plastic, in which the tank has a movable anchor that is horizontally extendable from the side of the tank.
2. Description of the Prior Art
An underground septic system generally includes a large settler/digester tank which is connected by pipe to the source of sewage, and a smaller distribution tank also called a distribution box or drop box that is connected by pipe to the settler/digester tank and to outlet pipes which extend over a large area called a leaching field where the treated sewage is deconcentrated by percolating it out over the large area.
Disruption of a pipe connection at a septic tank results in an unacceptable environmental hazard by overloading the soil locally with undigested sewage or treated effluent. Disruption of the piping can be caused by shearing off at the pipe connection with the septic tank if the tank moves within the soil.
Concrete septic tanks have a high weight-to-volume ratio that benefits stable underground installation. But, they are costly in time and equipment to make, to ship from the caster to the installation site, to handle, and to install in the ground.
A plastic septic tank is relatively inexpensive and quick to make, which saves manufacturing and inventory cost, takes less time and lower cost equipment to ship from the molder to the installation site, to handle, and to install in the ground.
Less soil needs to be removed to make room for a plastic tank than a concrete tank of equal volume because the walls of the plastic tank are thinner and overall volume is less.
The light weight of a plastic tank causes two stability problems when the tank is installed in the earth.
One problem is that the full-to-empty overall weight ratio is high, resulting in a higher full-to-empty downward settling force ratio for the plastic tank than for a concrete tank of equivalent liquid volume.
Another problem is that a partially empty tank may be forced out of the earth by a rise in water level in the soil from heavy rain, as buoyancy of the tank overcomes the weight of the tank and grip of the tank by the soil.
U.S. Pat. No. 3,904,524, patented by Pelton et al., Sep. 9, 1975, describes several items and methods for attacking the problem in a light weight oil separator tank adapted for underground burial that is made with a foam and fiberglass reinforced skin over a frame.
In one method, a standpipe mounted below a covered observation opening in the top of the tank extends through the bottom of the tank. Height of ground water in the pipe provides visible indicia of the buoyant force which the soil water may impose against the bottom of the tank.
In another method, a divider wall within the tank is made higher to increase the height of water in a separating chamber in the tank to increase the downward force within the tank.
In another method, ground water is allowed to come into the tank by rising up the standpipe when the ground water pressure is otherwise sufficient to force the tank out of the ground, to increase the downward force within the tank.
In another method the bottom wall of the tank extends fixedly horizontally beyond the tank on all four sides of the rectangular tank to act as an anchor in the soil. Gussets are interposed between vertical studs of the frame and the horizontal bottom wall during construction of the tank to strengthen the construction.